The US space agency said fixing the ripped solar array has become the top priority for the Discovery shuttle mission because without it there was a risk the tear could spread and render the power-generating wing useless.

Astronauts Scott Parazynski and Doug Wheelock emerged into open space shortly after 6:00 am (1000 GMT), the National Aeronautics and Space Administration (NASA) said.

Parazynski, 46, an experienced spacewalker, is carrying out most of the unprecedented and dangerous repair job.

After making his way across the station's truss structure, he attached himself to a 15-meter (49 feet) extension boom attached to the space station's 18-meter (59 feet) robotic arm and began a ride to the damaged panel.

Attached to the boom by his feet, the astronaut will try to loosen the snagged wire that caused the tears on the two solar panels without touching the array which carries some 300 volts, according to NASA.

"If he can clear that snag without actually cutting the cable or the guide wire, I expect it to be a quick fix," ISS flight director Derek Hassmann told reporters on Friday.

Then Parazynski will try to sew the wing back together by inserting tabs or "cuff links" through the holes in the array, which NASA hopes will allow it to be fully extended without further damage.

"I would expect if he does not have to do any cutting... I think we'll be done in 30 minutes at the work-site," Hassman said.

"And even then I expect that two hours and 30 minutes at the work-site will be plenty of time to do what we need to do."

NASA says the solar array, one of three on the space station, is critical to providing extra electricity for planned European and Japanese science labs.

The European Columbus laboratory is due to be delivered to the ISS in December and the Japanese Kibo lab in April 2008.

Wheelock will accompany Parazynski from the station's truss while mission specialist Stephanie Wilson aboard the shuttle and engineer Dan Tani on the ISS will maneuver the robotic arm.

Italian mission specialist Paolo Nespoli will serve as the coordinator for the spacewalk.

Parazynski will have to avoid touching the electrified solar panels and rely entirely on tools that were rounded up on the shuttle.

The risk of electrocution was real but NASA had done "everything reasonable to keep that from happening," David Wolf, head of spacewalk training at NASA's center in Houston, told a news conference.

Wolf said that "an electrocution is conceivable but extremely unlikely," adding that in such a case the astronaut would not be burned but could receive a "mild shock."

Carrying out such a complex task in a cumbersome space suit also presented a problem, he said. Working with the stiff spacesuit gloves "is like sewing with mittens on."

Compared to astronauts on previous spacewalks, Parazynski also will be farther out from the shuttle -- an entire hour from safety instead of the customary 30 minutes if he needs to end the walk in an emergency.

If all goes well, Discovery will undock from the space station on Monday and return to Earth on Wednesday. It blasted off from Earth on October 23.

The spacewalk on Saturday has overtaken previous plans to examine a malfunctioning rotary joint that turns the solar arrays.

In an earlier spacewalk, small metal shavings were found in the joint which showed unusual wear.

The shuttle Discovery brought to the ISS a huge bus-size module called Harmony, which will eventually connect together the Japanese and European laboratories to be delivered to the station in the coming months.

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